The present invention relates to an ink-jet head for use in an ink-jet printer, and more particularly, it relates to improvement of a vibration plate of a piezoelectric actuator used for discharging ink in an ink-jet head.
Recently, ink-jet printers are widely used in offices and households. Various systems have been proposed for ink-jet heads used in the ink-jet printers in order to meet recent demands for low noise and high print quality. In general, the systems for the ink-jet heads can be roughly divided into the following two systems:
In a first system, part of an ink passage and an ink chamber is formed into a pressure chamber by using a piezoelectric actuator having a piezoelectric device, and a pulse voltage is applied to the piezoelectric device so as to deform the piezoelectric actuator. Thus, the pressure chamber is deformed to have a smaller volume, thereby generating a pressure pulse within the pressure chamber. By using the pressure pulse, ink drops are jetted through a nozzle hole communicating with the pressure chamber.
In a second system, an exothermic resistance is provided in an ink passage, and a pulse voltage is applied to the exothermic resistance so as to generate heat therein. Thus, ink contained in the passage is boiled with vapor bubble generated. By using the pressure of the vapor bubble, ink drops are jetted through a nozzle hole.
The present invention relates to an ink-jet head of the first system, and hence, this system will be further described in detail. FIGS. 11 through 13 show an exemplified conventional ink-jet head of the first system, and the ink-jet head comprises a head body 101 including a plurality of pressure chamber concaves 102 each having a supply port 102a for supplying ink and a discharge port 102b for discharging ink. The concaves 102 of the head body 101 are arranged along one direction at predetermined intervals.
The head body 101 includes a pressure chamber part 105 forming the side walls of the concaves 102, an ink passage part 106 forming the bottoms of the concaves 102 and including plurality of thin plates adhered to one another, and a nozzle plate 113. Within the ink passage part 106, an ink supply passage 107 communicating with the supply port 102a of each concave 102 and an ink discharge passage 108 communicating with the discharge port 102b of each concave 102 are formed. Each ink supply passage 107 communicates with an ink supply chamber 110 extending in the direction of arranging the concaves 102, and the ink supply chamber 110 communicates with an ink supply hole 111 formed in the pressure chamber part 105 and the ink passage part 106 and connected with an external ink tank (not shown). In the nozzle plate 113, nozzle holes 114 respectively connected with the ink discharge passages 108 are formed.
On the upper surface of the pressure chamber part 105 of the head body 101, a piezoelectric actuator 121 is disposed. The piezoelectric actuator 121 includes one flat vibration plate 122 that covers all the concaves 102 of the head body 101 so as to form pressure chambers 103 together with the concaves 102. The vibration plate 122 also works as a common electrode shared by all piezoelectric devices 123 described below. Also, the piezoelectric actuator 121 includes the piezoelectric devices 123 disposed on the vibration plate 122 respectively correspondingly to the pressure chambers 103 and individual electrodes 124 respectively disposed on the piezoelectric devices 123 for applying a voltage to the piezoelectric devices 123.
When a pulse voltage is applied between the vibration plate 122 serving as the common electrode and each individual electrode 124, each piezoelectric device 123 shrinks in a lateral direction perpendicular to a thickness direction, but the vibration plate 122 and the individual electrode 124 do not shrink. Therefore, a portion of the vibration plate 122 corresponding to the piezoelectric device 123 is deformed into a convex projecting toward the pressure chamber 103 due to so-called the bimetal effect. This deformation causes a pressure within the pressure chamber 103, and owing to the pressure, ink contained in the pressure chamber 103 is jetted from the nozzle hole 114 through the discharge port 102b and the ink discharge passage 108.
In the ink-jet head for jetting ink by using the piezoelectric actuator as described above, various improvements have been recently made so as to meet strict demands for compactness and light weight, a low driving voltage, low noise, low cost, and high controllability in jetting ink. In order to attain further compactness and higher performance, the vibration plate, the piezoelectric devices and the like can be formed from thin films easily subjected to refined processes.
When the vibration plate and the piezoelectric devices are simply made from thin films with keeping the shape and the structure of the conventional piezoelectric actuator, however, there is a fear of occurrence of cracks and film peeling in the vibration plate and the piezoelectric devices in manufacture of the ink-jet head. As a result, the productivity of the ink-jet head is disadvantageously lowered. Accordingly, an ink-jet head that is not only compact but also can be easily manufactured with good productivity is desired.
The present invention was devised under these circumstances. An object of the invention is, in an ink-jet head for jetting ink contained in a pressure chamber by using a piezoelectric actuator, changing the structure of a vibration plate of the piezoelectric actuator so as to make compact the ink-jet head by forming a vibration plate and piezoelectric devices from thin films as well as to improve the productivity as far as possible by suppressing occurrence of cracks and the like in the vibration plate and the piezoelectric devices.
In order to achieve the aforementioned object, according to the invention, vibration plates are separately provided correspondingly to one or plural pressure chambers, or a portion of a vibration plate corresponding to each pressure chamber is bent into a convex projecting in an opposite direction to the pressure chamber.
Specifically, the ink-jet head of this invention comprises a head body including plural pressure chamber concaves each having a supply port for supplying ink and a discharge port for discharging ink; and a piezoelectric actuator including mutually electrically connected vibration plates that cover the concaves of the head body in order to form pressure chambers together with the concaves and are separately provided correspondingly to one or plural pressure chambers; piezoelectric devices respectively provided on surfaces of the vibration plates opposite to the pressure chambers correspondingly to the pressure chambers; and individual electrodes respectively provided on surfaces of the piezoelectric devices opposite to the vibration plates for applying a voltage to the piezoelectric devices together with the vibration plates, wherein the vibration plates are deformed in a manner that volumes of the pressure chambers are decreased by applying a voltage to the piezoelectric devices through the vibration plates and the individual electrodes, whereby ink contained in the pressure chambers is discharged through the discharge ports.
In this manner, the vibration plates are separately provided correspondingly to one or plural pressure chambers. Therefore, the separated vibration plates can be prevented from affecting one another in their internal stress and strain, and hence, the internal stress caused in each vibration plate can be reduced as compared with the case where one vibration plate covers all the concaves of the head body. As a result, the internal stress caused in the piezoelectric device and the individual electrode disposed on the surface of the vibration plate opposite to the pressure chamber can be also reduced. Accordingly, even when the vibration plate, the piezoelectric device and the like are formed from thin films, the occurrence of cracks and film peeling in the vibration plate, the piezoelectric device and the like can be prevented in the manufacture of the ink-jet head, resulting in improving the productivity. Furthermore, the separated vibration plates do not mutually affect in their deformation in use of the ink-jet head, and hence, the mechanical strength of the vibration plate can be prevented from lowering, resulting in elongating the life of the ink-jet head.
In the ink-jet head, a portion of each of the vibration plates corresponding to each of the pressure chambers is preferably bent into a convex projecting toward an opposite direction to the corresponding pressure chamber.
Specifically, owing to a difference in the thermal expansion coefficient between the vibration plate and the piezoelectric device (which is larger in the vibration plate), the vibration plate shrinks more largely than the piezoelectric device but the piezoelectric device hardly shrinks. Accordingly, the vibration plate can be easily bent into a convex projecting toward the opposite direction to the pressure chamber. In this shrinkage, the vibration plate receives a tensile force from the piezoelectric device and the piezoelectric device receives a compressive force from the vibration plate, and hence, compressive internal stress is caused in the piezoelectric device. Thus, the occurrence of cracks and the like particularly in the piezoelectric device very weak against a tensile force can be more effectively suppressed. Furthermore, in use of the ink-jet head, due to the shrinkage of the piezoelectric device, the portion of the vibration plate corresponding to the pressure chamber is deformed to have a smaller convex dimension toward the opposite direction to the pressure chamber. Therefore, the piezoelectric device receives a tensile force from the vibration plate. However, since the compressive force is initially caused as described above, the tensile force and the compressive force cancel each other, so as to make the stress caused in the piezoelectric device comparative small. In addition, even when the deformation is small, the power for jetting ink can be larger than in the case where the vibration plate is in the shape of a flat plate or bent into a convex projecting toward the pressure chamber. Therefore, large deformation is not necessary. Also in consideration of this small deformation, the stress caused in deforming the vibration plate and the piezoelectric device can be reduced.
When the vibration plate is bent as described above, the convex projecting toward the opposite direction to the pressure chamber preferably has a maximum dimension of 0.05 through 10 xcexcm.
When the maximum convex dimension is smaller than 0.05 xcexcm, an effect to suppress failures of the vibration plate and the piezoelectric device during manufacture and use of the ink-jet head cannot be sufficiently exhibited. When the maximum convex dimension is larger than 10 xcexcm, cracks can be more easily caused in the vibration plate and the piezoelectric device during the manufacture on the contrary. Accordingly, the maximum convex dimension is preferably set to 0.05 through 10 xcexcm. In this manner, the productivity of the ink-jet head can be maximized.
Alternatively, the ink-jet head of this invention comprises a head body including plural pressure chamber concaves each having a supply port for supplying ink and a discharge port for discharging ink; and a piezoelectric actuator including a vibration plate covering the concaves of the head body in order to form pressure chambers together with the concaves and having a convex in a portion corresponding to each of the pressure chambers, the convex projecting toward an opposite direction to the corresponding pressure chamber; piezoelectric devices provided on a surface of the vibration plate opposite to the pressure chambers respectively correspondingly to the pressure chambers; and individual electrodes respectively provided on surfaces of the piezoelectric devices opposite to the vibration plate for applying a voltage to the piezoelectric devices together with the vibration plate, wherein the vibration plate is deformed in a manner that volumes of the pressure chambers are decreased by applying a voltage to the piezoelectric devices through the vibration plate and the individual electrodes, whereby ink contained in the pressure chambers is discharged through the discharge ports.
Owing to this structure, even when the vibration plate, the piezoelectric device and the like are formed from thin films, the occurrence of cracks and the like in the piezoelectric device in particular can be suppressed in the manufacture of the ink-jet head because compressive internal stress is caused in the piezoelectric device as described above. In addition, in use of the ink-jet head, the stress caused in deforming the vibration plate and the piezoelectric device can be reduced, resulting in elongating the life of the ink-jet head.
Also in this ink-jet head, the convex of the vibration plate projecting toward the opposite direction to the corresponding pressure chamber preferably has a maximum dimension of 0.05 through 10 xcexcm for the same reason as described above.